This article deals with the key issue of determination of foreign law substance by the arbitration court. We find out a common features and differences in foreign law substance determination by state courts and arbitration. The article studies the practice of foreign law substance determination by the different legal systems states and allocation of duties to determinate substance between court and parties. The article also analyzing the existent mechanisms of foreign law substance determination involving states bodies and experts. The attention is focused on the consequences of no determination or wrong determination of foreign law substance.